1. Field of the Invention
The present invention generally relates to an apparatus for detecting a short circuit in a load that is connected to a secondary winding of a transformer.
2. Problem to be Solved
Resonant transformers are typically used in lighting systems. Resonant transformers usually have a relatively high turn ratio. As a result of this high turn ratio, the impedance of the leakage inductor between the primary winding and the secondary winding is relatively high. When a switching frequency is applied to the transformer that is relatively higher than the resonant frequency (inductive mode), the impedance of the leakage inductor is generally the same as the impedance of the transformer""s output load. Due to the high impedance of the leakage inductor, the primary current of the transformer will not increase much more even if the output load is shorted. The conventional technique of detecting the amplitude of the primary current cannot determine if the output load is shorted.
It is therefore an object of the present invention to provide a unique apparatus and method for detecting a short circuit in a load connected to the secondary winding of a transformer.
Other objects and advantages of the present invention will be apparent to one of ordinary skill in the art in light of the ensuing description of the present invention.
In one aspect, the present invention is directed to an apparatus for detecting a short in a load which receives power from a transformer wherein the transformer has a primary winding and a secondary winding and wherein the load is connected to the secondary winding. The apparatus comprises a waveform conversion circuit for converting voltage and current signals representing the transformer primary winding voltage and current signals, respectively, into generally squarewave voltage and current signals, respectively. The apparatus further comprises a phase detector circuit for outputting a control signal having a first state when the generally squarewave voltage and current signals are substantially in phase and a second state when there is in a difference in phase between the generally squarewave voltage and current signals that is greater than a predetermined phase difference. The apparatus further comprises a filter for filtering the signal outputted by the phase detector circuit. In one embodiment, the apparatus further comprises a circuit for deactivating the load when the control signal has the second state. In one embodiment, the apparatus included means for indicating that a short exists in the load when the control signal has the first state.
In a related aspect, the present invention is directed to an apparatus for detecting a short in a lighting system which receives power from a transformer wherein the transformer has a primary winding and a secondary winding and wherein the lighting system is connected to the secondary winding. The apparatus comprises means for converting voltage and current signals representing the transformer primary voltage and current signals, respectively, into generally squarewave voltage and current signals, respectively, means for generating a signal having a first state when the generally squarewave voltage and current signals are substantially in phase and a second state when there is in a difference in phase between the generally squarewave voltage and current signals that is greater than a predetermined phase difference, and means for filtering the signal outputted by the phase detector circuit.
In yet a further aspect, the present invention is directed to a method for detecting a short in a load that receives power from a transformer that has a primary winding and secondary winding wherein the load is connected to the secondary winding. The method comprises the steps of providing voltage and current signals that represent the transformer primary voltage and current signals, respectively, converting the signals that represent the transformer primary voltage and current signals into generally squarewave voltage and current signals, respectively, generating a signal having a first state when the generally squarewave voltage and current signals are substantially in phase and a second state when there is in a difference in phase between the generally squarewave voltage and current signals that is greater than a predetermined phase difference, and filtering the signal outputted by the phase detector circuit.